The present invention relates generally to optical coating chambers, and more particularly to a shutter apparatus for an optical coating chamber viewport.
The spatial and temporal film thickness variations of precision coatings, such as multilayer antireflection (AR) coatings, generally must be held to within about plus or minus one percent to maintain the coatings' optical properties. For economical production, it is desirable to apply coatings in a large area, in-line sputtering apparatus about one meter or greater in width.
Large area commercial coating applications, such as the application of thermal control and antireflection coatings to architectural and automobile glazings, most often use DC reactive sputtering as a deposition process. The present invention may be used in connection with DC reactive sputtering as well as with other kinds of deposition processes.
In the DC reactive sputtering process, the articles to be coated are transported through a series of in-line vacuum coating chambers isolated from one another by vacuum locks. Each coating chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. The cathodes may be rectangular or cylindrical and are typically 0.10 to 0.30 meters wide and a meter or greater in length. A layer of material to be sputtered is applied to the cathode surface. This surface layer or material is known as the target or target material.
Inside the coating chambers, a sputtering gas discharge is maintained, for example, at a partial vacuum pressure of about 3 millitorr. The sputtering gas comprises a mixture of inert gas, such as argon, with a small proportion of reactive gas, such as oxygen, for the formation of oxides. Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target on a conveyor mechanism, such as rollers. The atoms react on the substrate with the reactive gas in the sputtering gas discharge to form a thin film on the substrate.
The architectural glass coating process was made commercially feasible by the development of the magnetically-enhanced planar magnetron. This magnetron has an array of magnets arranged in the form of a closed loop and mounted in a fixed position behind a target. A magnetic field in the form of a closed loop is thus formed in front of the target. The magnetic field traps electrons from the discharge and causes them to travel in a spiral pattern. This creates more intense ionization and higher sputtering rates. The planar magnetron is described in U.S. Pat. No. 4,166,018.
While effective for some coating applications, such as thermal control, the magnetically-enhanced planar magnetron had problems with others, such as high precision AR coatings. Even the simplest AR coating is twice as thick as a thermal control coating. Therefore higher deposition rates are required to obtain a comparable production cost between the two types of coatings. Additionally, AR coatings require a low refractive index material, such as silicon dioxide, as an outer film. At high deposition rates, and with accuracy and long term stability, this material is extremely difficult to deposit using DC reactive sputtering processes. Particularly, thickness variations tend to occur in the outer film, causing perceptible color performance variations.
The rotary or rotating magnetron was developed to overcome some of the problems inherent in the planar magnetron. The rotating magnetron uses a cylindrical cathode. The cathode is rotated continually over a magnetic array which defines the sputtering zone. As such, a new portion of the target is continually presented to the sputtering zone which eases cooling problems and allows higher operating powers. The rotation of the cathode also ensures that the erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
In addition to the sputtering and conveyor apparatus, optical coating chambers are also equipped with an opening or openings in their walls containing a transparent material, such as glass or plastic. This "viewport" permits an outside observer to view the coating process as it is occurring inside the chamber. The opportunity to view the sputtering process through a viewport can be important in ensuring that the process is taking place properly.
During the coating process, some amount of coating material is deposited other than on the substrate as a natural consequence of the sputtering method. As sputtering progresses, this off-substrate coating material begins to deposit on the coating chamber walls and other interior components of the coating chamber, including the interior surface of the viewports. Over time, the built-up coating material on the interior surface of the viewports obscures the view into the chamber. This necessitates regular removal and cleaning or replacement of the viewports, resulting in chamber down-time and increased production costs.
Accordingly, an object of the present invention is to provide a shutter apparatus capable of protecting the interior surface of an evacuable optical coating chamber viewport from the deposition of off-substrate coating material when closed, and exposing the viewport surface to permit viewing through the viewport into the coating chamber when open.
It is a further object of the present invention is to provide a shutter apparatus for protecting the interior surface of a viewport from the deposition of off-substrate coating material wherein the shutter may be opened and closed by means that allow the vacuum inside the chamber to be maintained, such as magnetic coupling of the shutter to an actuator outside the chamber.
It is yet another object of the present invention to provide a shutter apparatus the may be quickly and easily installed on a conventional optical coating chamber viewport.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.